There is a variety of devices and mechanisms for deploying implantable medical devices in a lumen or other organ of a patient. For instance, in the case of stents and stent grafts, the device may be of a self-expandable type, in which case the device will expand by its own expansion force from a compressed delivery configuration to its expanded deployed condition. In order to ensure satisfactory anchoring to the lumen wall, the device typically has an expanded diameter which is larger than the diameter of the lumen. Thus, the device will maintain a constant expansion force against the lumen. This is not always considered advantageous.
Another arrangement provides a device which does no exhibit its own expansion force but is expanded by a separate mechanism, which is typically part of the introducer assembly. Such devices, for instance stents or stent grafts intended to be deployed in the aorta, can thus be expanded to a chosen diameter, typically the normal diameter of the aorta, and retain that diameter without imparting on the walls of the lumen any further expansion force which can be detrimental to the integrity of the lumen. A common mechanism to expand such devices is by means of a balloon catheter. Such a device provides a balloon at the end of the catheter, over which the radially compressed medical device is located. Inflation of the balloon expands the device to its deployed configuration.
There have been various disclosures of balloon catheters in the prior art. For instance, U.S. Pat. No. 5,868,776 discloses a balloon catheter having a double balloon structure provided with an inner balloon and an outer balloon. The inner balloon is made of a non-compliant material while the outer balloon is made of a stretchable material which is intended to provide a superior restoring force to aid in the deflation of the inner balloon, by providing a compressing force on the inner balloon, and it is explained that the outer balloon may always be taut.
U.S. Pat. No. 6,187,014 discloses a stent deployment device using a catheter with a double-layered balloon. The outer balloon is made of a compliant material whereas the inner balloon is made of a substantially non-compliant material. The structure provides for the outer, softer, balloon to be inflated first so as to deploy a stent. The outer balloon is then deflated, before the inner balloon is inflated. The inner balloon, which is stronger, is intended to set or fix the stent.
An advantage of balloon expandable medical devices, more particularly medical devices which are expanded by a separate expansion mechanism, is that they can provide a reliable and effective prosthesis which does not impart of the human body artificial stresses.
It is important with balloon expandable medical devices to be able to expand these to a predetermined diameter and it is equally important to be able to expand these in a manner such that they attain a properly rounded shape. In order to achieve this, the primary inflation balloon is typically made of a substantially non-compliant material. Such a material can provide the strong expansion force necessary to expand the medical device and also provides a reliable and predictable expanded shape, necessary to ensure proper deployment of the medical device.
This optimal characteristic of expansion balloons, however, involves a compromise in that the lumens of different patients vary in diameter. For instance, the aorta of an adult typically varies from around 22 mm in diameter to around 38 or 40 mm. This causes difficulties in the selection and provision of a suitable balloon. For instance, if a balloon is chosen which is larger than the diameter of the lumen, the balloon will not be fully expanded by the time the medical device has been expanded to the walls of the lumen and thus the balloon will not have attained its fully expanded state. Given the non-compliant nature of the balloon and the fact that it is typically wrapped onto the introducer for deployment, it is not possible to assure that the balloon has acquired its rounded configuration and that it will thus impart a full and complete rounded expansion force to the medical device. In such an event, it cannot be assured that the medical device will be properly deployed, that is pressed against the lumen wall across the entirety of its circumferential extent. This can therefore lead to less than optimum anchoring of the medical device to the lumen wall, with possible risks of migration, and can fail to provide patency of the device to the lumen wall, that is a proper seal of the device to the lumen wall, important for instance in the case of stent grafts.
On the other hand use of a balloon which has a diameter which is smaller than the lumen wall can lead to insufficient expansion of the medical device, inadequate anchoring and also inadequate sealing of the device to the lumen wall.
One method which can be used to seek to avoid these problems is to expand the medical device in stages, that is to start with a smaller diameter balloon, then to introduce a larger balloon into the lumen to expand the device further, continuing until the right sized balloon finally expands the device to the required diameter. It will be appreciated that this entails a multi-step expansion stage and thus a more elaborate deployment operation compared to a single stage operation.
Whilst in theory it might be possible to try to preselect a balloon of the correct diameter, in practice this is not always possible in light of the difficulties of being able to obtain an accurate measure of the internal diameter of a patient's lumen.
There is currently no structure which can resolve these issues.